Theres no logic in flowering a plant less then 18"...to get what...a joint ? I doubt that sex has even shown at this juncture.. Try to throw a 46w cfl in the mix (if none) and get some vegetation.. As for topping...its a moot point on a small plant if your plans are to harvest quickly..I topped mine at 6"...now the offshoots are about 20' long..topping work only if you let it grow to its peek height...topping is a good plan for the next crop..It's just that to take the time to germ., water, and grow the plant , you want maximum output...... IMO
Luck...whatever your choice..